Electric field heat-treatment of liquids



J 15, 1952 J. w. ROBERTSON 2,582,281

ELECTRIC FIELD HEAT TREATMENT OF LIQUIDS Filed April 12, 1946 gvwwwfobfab/1 izobertison 5A MW:

Patented Jan. 15, 1952 ELECTRIC FIELD HEAT-TREATMENT OF LIQUIDS John W.Robertson, Englewood, N. J assignor, by direct and mesne assignments, ofone-half to Ellis-Foster Company,

a corporation of New and one-half to Montclair Research Corporation,acorporation of New Jersey Application April 12, 1946, Serial No.661,793

7 Claims.

This invention relates to the heattreatment of liquids and apparatustherefor and is particularly concerned with the utilization of highfrequency or radio frequency power applied in the heat treatment ofliquid materials.

While radio frequency power for heating dielectric types of materials byplacing the material between condenser plates has developed in recentyears, little has been done in the develop ment of the utilizationof'such types of heating with liquids, particularly to utilize thevarious factors which enter into such operations in the eflicienttreatment of liquid materials.

Among the objects of the present invention is the utilization of highfrequency or radio frequency power in the heat treatment of liquids.

Further objects include the utilization of such methods and apparatustherefor in the treatment of liquids while the latter are in the form ofa thin film.

Still further objects and advantages of the present invention willappear from the more detailed description set forth below it beingunderstood that such detailed description is given by way ofillustration and explanation only, and not by way of limitation, sincevarious changes therein may be made by those skilled in the art, withoutdeparting from the scope and spirit of the present invention.

In connection with that more detailed description, there is shown in theaccompanying drawings, in

Figure 1. an elevation in section of one form of apparatus that can beutilized in carrying out the present invention; in

Figure 2, a plan view of the apparatus of Figure 1; and in Figure 3, amodified form ofapparatus shown in section, that may be utilized incarrying out the present invention.

In accordance with the present invention, liquids of any desired typebut: particularly those which are relatively poor conductorselectrically, are passed as a thin film through a heating zone in whichthey are subjected to high frequency or radio frequency power in orderto heat the liquid for any desired purpose.

In carrying out the heating operations inaccordance with this invention,the heating effects obtained by the use of a high voltage electrode inproximity to a liquid surface is employed. It has been found that theheating effects on liquids of varying conductivity indicates that vaporsabove the surface of the liquid may behave in much the same manner assharp edges or points of metal in initiating corona discharges (visibleor invisible). So that when such surface is at high potential, thesedischarges start (usually invisibly) and cause rapid heating of thesurface as they discharge into the air. The heating of such liquids iscomplicated under the stated conditions by various factors including theeffect of displacement currents (true dielectric loss), conductioncurrents (resistance loss), skin effect" phenomena, corona discharges,and possibly other unknown effects. In some cases the corona effects areno doubt present during the conditions which produce skin effectheating. Utilization of these phenomena has been found quite useful inquickly heating thin films of liquids for various purposes, as forexample, for evaporation, distillation, pasteurization, producingchemical reactions, drying, etc.

The present invention utilizes forces which come into play at highervoltages, particularly those above 5000 volts, and more particularlyabove 10,000 volts and more specifically when these voltages are appliedto materials which are neither dielectrics nor conductors in the bestknown sense of those words. In other words the stated materials will notheat effectively as a dielectric because they are too good as conductorsand they will not heat effectively by induction because they are toogood as insulators.

One way that may be utilized herein to define the nature of conductorscan be based on the behavior of salt solutions. Poor conductors,"especially in the nature of salt solutions (or acid or alkali)comparable in dielectric and corona behavior with sodium chloridesolutions stronger than about .05 will be affected by the electricalfield or "corona discharge field selectively, faster than surroundingmedia of dielectric behavior such as that of sodium chloride solutionsbelow about 113% lished. References herein to corona discharge field,"invisible corona, invisible discharges and other corona phenomena arenot to the visible corona discharge which is familiar to all but ratherto the ionized field surrounding high voltage electrodes under suitableconditions and which apparently actually dissipates energy at levels toolow to discharge through a concentrated, visible path in the well knowncorona phenomenon.

Principally in connection with the present invention poor conductors aresubjected to treatment in a zone within the discharge field of highfrequency power where such poor conductors possibly act more as metallicparticles would and as set forth and further estab become focal pointsfor the initiation of such invisible discharges themselves, thus causingoverheating, and other effects which are not yet well understood butwhich are often instantaneous in their action. The principle in use isto cause concentration of the electrical field or discharge on particleswhich may be conductors or poor conductors in that field.

In carrying out the operations of the present invention, the effects ofa high voltage electrode in proximity to the object to be affected isemployed. For this purpose as stated, voltages of the order of 5000above ground potential are useful but for best effectiveness, voltagesof over 10,000 are employed. These voltages bring into play moreeffectively, the phenomenon referred to herein and which will bereferred to herein as corona discharge effect or invisible coronadischarge effect. In so doing, no limitation is intended as thephenomenon may be described in other ways by physicists. It has beennoted that at these voltages, visible corona discharges start from sharpmetallic points and, surprisingly enough, from the surface of aconductive liquid such as a salt or acid or alkali solution. It appearsthen that vapors above the surface of the liquid behave in much the samemanner as sharp edges or points of metal in initiating corona discharges(visible or invisible). These same solutions (all too conductive to heateffectively as a U dielectric) will also heat in a partially enclosedspace if placed in the vicinity of an electrode high potential andparticularly at frequencies above 1 me. Lower frequencies may be usefulat higher voltages but practical industrial considerations call forfrequencies of the order of 30 inc. and above and at such voltage that afield of desired strength and extent is made for the proposed use. Aspointed out above, effective use of the method is particularly concernedwith the voltages as described above and more desirably voltages above10,000 (electrode potential). It has been further noted that the efiectof increasing frequency is rather pronounced due possibly to the factthat higher frequencies, it is believed, tend to leave such terminalsurfaces as described above, more readily than lower frequencies. Thistendency to corona at lower voltages in the high frequencies may thus beemphasized in connection with the present invention and the increasingeffects of high frequencies in the disclosed method supports the coronadischarge theory. But as explained, the phenomena are not to berestricted by the use of such terms as corona discharge because definiteeffects have been obtained whatever may be the explanation therefor orwhether such explanation may lie along other lines.

While voltages have been particularly emphasized above with respect tothe effects obtained, it may be pointed out that frequencies of lessthan Sflmc. are effective but it is preferred to utilize frequenciesabove 30 me. It has been shown that salt solutions heat inductively withincreasing usefulness as frequency is increased but at frequencies below1000 me. this effect is not pronounced enough for certain uses. Me. isused to designate megacycle.

To explain phenomena involved herein, the following example showing therange of concentration of salt solutions which can be effectively heatedas a dielectric is given. Dielectric heating apparatus operating atabout mo. and 5000 volts between electrodes is set up so as to give asubstantially uniform dielectric field through high frequency or radiofrequency power.

a quartz test tube containing the solution to be studied. The timerequired to boil various concentrations of sodium chloride solutions wasdetermined and the relative heating rate of surface compared to body ofliquid was noted.

NaCl Gone. Time to boil l Comments None-distilled water. 15 O. rise in30 Uniform heating. mm. 240 seconds Do. 80 seconds.... D 47 seconds....Do. 40 seconds... Surfacchccts luster. 43 seconds... Do. 43 seconds Do.49 seconds.. Do. seconds.. Do. 215 seconds Surface heats faster andboils first 1,000 seconds D0.

From these results it is seen that salt solutions under 04%concentration exhibit rapid heating as dielectrics but those over 04% donot heat so well as dielectrics and in fact the surfaces of strongersolutions heat so much faster than the under surface liquid, thatboiling first takes place at the surface while the lower layers maystill be 10 or 20 below boiling temperatures, thus indicating that theeffective heating is not caused by dielectric heating but by other highfrequency effects in which invisible corona discharge plays an importantpart.

Similar charts on phosphoric acid, hydrochloric acid, sodium sulfate,and p-toluene sulfonic acid all show maximum heating rate as dielectricsat concentrations below 05%. It would be expected that there would beobtained much more rapid and selective heating when placed near anelectrode at high radio frequency voltage, than would take place at apoint say half way between two plates of a dielectric heating condenserat the same electrode voltage.

This behavior may be demonstrated by vertically suspending two flatplates, for example 5 inches square, as electrodes in a kerosene bath.Kerosene is a good dielectric and heats very slowly in a dielectricfield. With 10,000 volts across the plates, spaced at 8 inches, thekerosene will rise in temperature about 1 C. in 30 minutes. Five Pyrextest tubes filled with 0.1% salt solution suspended at equal spacings ona center line between the electrodes were subjected to the applicationof the radio frequency power. In five minutes the temperature of thesalt solution in the tubes nearest the electrodes had increased 15 C. atthe surface and 5 C. at the bottom, and the temperature in the tube halfway between the plates had not increased measurably.

The liquid to be treated may be conveyed or caused to pass over adielectric surface or body having such surface where it is subjected tosuch The nature of the material undergoing treatment, the nature of thesurface over which it flows, and the positioning of the electrodes whichapply the power from a conventional type of radio frequency generatorsystem, determine the type of heating effects which are obtained. Thusthe thin film of liquid may be the chief resistance (considering anymedium in which heating takes place) between electrodes properlypositioned and the film heated rapidly due to its resistance when asuitable voltage and frequency is impressed between the electrodes. Orthe positioning of the electrodes may be so effected that bothresistance effect and dielectric effect are employed in producing theheating. If the voltage impressed at the surface of the film is highenough. corona discharge may also take place and afiect the heatingwhich is carried out.

The resistance and corona discharge effects may be augmented by varyingthe surface of the area covered by the thin liquid film. Thus if suchsurface is covered completely or discontinuously with a thin metal filmsuch as of silver, platinum, etc., the film may be utilized as anelectrode and serve as such, and especially at a high voltage, willcause an increase in corona discharge effect.

It is further possible to increase the evaporation rate from suchsurface or otherwise affect the heating function obtained by variationin the character of the surface employed. For example, such surface overwhich the liquid film is flowing may be made of a woven glass cloth. Anirregular surface may thus be produced or in other ways. It is not clearwhether this improvement is due to the irregular surface of the clothoffering more protruding points for corona discharge or more paths forresistance currents. But at any rate in such utilization, as forexample, a glass cloth as the surface over which the thin film of liquidfiows, proves to be a useful procedure under many conditions whether oneor two electrodes are employed. There particularly may be use for otheruneven surfaces of large area in small volume of space employing suchmaterials as sintered glass or other ceramic materials.

Such cloth or other uneven surface may furthermore be metallized withone of the metals as set forth above for the purposes there described toproduce a double effect and also where chemical reactions are takingplace, such metal may act catalytically.

Furthermore these operations may be carried out within a closed chamberin order to control the type of atmosphere in which the heat treatmenttakes place. In such closed chamber the atmosphere may include an inertgas or vapor with respect to the liquid undergoing treatment or on theother hand, a gas or vapor which reacts chemically with said liquid maybe introduced into such closed space to contact with the liquid duringthe heat treatment. Where chemical reaction is being employed, unreactedgas or vapor from the heating zone may be withdrawn and recycled to saidzone.

Also by the utilization of such enclosed chamber within which theapparatus is employed, the pressure under which the heat treatment iscarried out may be controlled and either vacuum of any desired degreemay be employed, or superatmospheric pressures may be maintained duringsuch heat treatment. The apparatus may under such circumstances beutilized for distillations and where a very high vacuum is employed,molecular distillation may be carried out employing a suitable spacingbetween electrode area and outside walls.

The liquid to be treated in the form of athin film as set forth abovemay first be subjected to heating to raise its temperature to a point atwhich such heat treatment in the thin film condition may desirably becarried out. For such purpose a body of the liquid may be heated by highfrequency or radio frequency power dielectrically and the liquidwithdrawn from such heated body in the form of a thin film and subjectedfor treatment as explained above.

One form of apparatus utilizing these features is illustrated in Figures1 and 2. As there shown, a container or vessel I for the liquid to betreated contains a body of such liquid 2, the inlet 3 servtrode II. 55'

ing to introduce such liquid into the container I. The feed of suchliquid through the inlet 3 may be either continuous or discontinuous.Such vessel or container I while illustrated as cylindrical, may takeany desired shape and may be oval, octagonal, etc. Attached at l to theupper end of the container I is a downwardly .depending dome shapedmember 5 over which the liquid from container I may overflow in the formof a thin film where it is subjected to the heat treatment in accordancewith the present invention. The liquid which flows over the dome shapedsurface 5 may be received in a trough member 6' attached to the outerperipheral edge of the dome shaped member 5 and removed through theoutlet 1 connected to such trough 6. The dome shaped surface 5 may becurved or straight and at almost any angle and the distance over whichthe thin film of liquid fiows on the surface 5 may be any desireddistance within the limits of processing equipment.

.Desirably a hood of either glass or metal dependnig upon the particularutilization, as shown at 8 may be positioned over the dome shaped member5, desirably equidistant from it at all points. This hood member 8 mayserve as a condensing area for vapors which are to be collected afterseparation from the body of the liquid. The hood 8 may also at itsperipheral edge have a trough portion 9 for collection of the liquidswhich condense from the vapors in contact with the hood member 8 andsuitably withdrawn through any desired outlet from such trough 9. Anoutlet I I] may also be supplied for removal of vapors which may bevalved.

In the form of apparatus shown in Figures 1 and 2, electrodes withproper connections to a source of radio frequency power are employed forproducing an initial dielectric heating of the body of liquid in thecontainer I and also heating of the thin film of liquid which overflowsfrom the liquid 2 in container I along the dome shaped surface 5. Onearrangement of electrodes that may be utilized is illustrated in Figures1 and 2. As there shown, the bottom of the container I is placedadjacent to an electrode II, and the container I may rest on theelectrode I I, or be spaced therefrom with an air spacing between, or ablock of insulating material may be placed between the electrode I I andthe bottom of the container I.

To overcome localization of heating or possible overheating oi thebottom of such vessel, the container I is desirably spaced from thisplate elec- Air spacing may be employed, but under commercial conditionsmay offer mechanical difficulties. When the container I is placedadjacent to such plate electrode I I, as for example, in being placeddirectly on such electrode, it is found desirable to utilize a materialof low power loss factor between the bottom of the vessel or container Iand the plate electrode II. Such low power loss factor material isindicated at I2. The type of material to be used depends on theparticular conditions of the operation being carried out. A block ofsintered Pyrex glass of suit able thickness is fairly useful where theconditions are not such that there is rapid heating of such block ofmaterial. More desirably a material of lower loss factor may be employedsuch as sintered or porous Nonex glass or Vycor (96% silica) glass orquartz or volcanic rock or pumice stone or fibers of these materials. Asimple means that may be employed, particularly in the heat treatmentof'smaller vessels or contain- 7 ers, consists inemploying-apiece-of"Mycalex (mica and glass) sheet together with"a'i'o'lded piece of glass cloth to provide added spacing with a largepercentage of-a-ir insulation. Such insulation between the container Iand the plate electrode II sufficiently controls the hea'ting operationswith respect to the container; I and-electrode II.- This bottomelectrode I'I- may be grounded, depending' upon the effect beingutilized and the design ofthe radio frequency ap-' h paratus.

A second ring electrode I3 encircles the eon-- tainer I and ispositionedatadistance fromthe" plate electrode II. The utiliaatio'n ofthe ring and plate arrangementas described enables rea'dy control of theheatingefiectofthebodyof liquid-2 in container 1 to be carried out.fi-he plateelec trode II may be a 'flatcircular plate et metal while thering electrode 13 is alsoo'f metal. Desirable connections are made tosupply' 'the power to such electrodes from a conventionalradiofrequencypower source. The ring electrode I3 may be positioned atvariouspoints distantf rom theelectrode I l to control theheating'eiiect obtained. instead of an arrangement of'a ringlelectrodeand plate electrode as described, two adjustable rings may be used, eachsuch ring electrode-encircling the container l. The positioning ofthe-ring electrade it with respect to the materialundergoing heattreatment must be taken-into account and are highly important inprioducingthe particular results desired. In order'to obtain the bestfield distribution for uniform heating, the ring should be as high aspossible, evenabove'the surface of the batch of material undergoingtreatment. But this is undesirable in those cases where too much surfaceheating. takes place and insuch cases the ring should be'lowered untilitis:inaposition as illustrated, all ofthe factors-involved depending;on the conductivity of the batch and irability of heating oneportion ofthe body" more rapidl than another.

If it is desiredto heat one portion of the body of material-Z incontainer I morerapidly thanother portions thereof, .expedientsmay be employed for that purpose. Thus/blocks of high-dielectric less material,especially of'larger'surface' such as sintered; high loss gl'ass'mayb'epositioned in the container at the point wherethe greatest heatingeiiect is desired. Byadjustment of the fact just discussed above, almostany reasonable degree of control of temperature can be obtained forgiven processingor treatment, especially for liquids and homogeneousmaterials. And even for non-homogeneous.niaterials under-'- goingtreatment, the control ofitemperature-may" be readily carried out by'theexpedient-set iorth'.

A third electrode 14 is shown positioned'u'nderthe outer periphery ofthe dome shaped surface under the lower edge of that overflow area. iiiis electrode It may be spaced from ori in contact with the materialof'theoverflow area and may in fact be placed on top of the dome-shapedmember .3 and in contact with or above theliquld film. The powerconnection to this electrode I4 made conventionally in a manner notshown.

The liquid to be treated, entersthe'bottomol' the container 5 from theinlet 3, fills the container i at any suitable speed, and passesupwardly past the ring electrode I3. At this point we have heating inthis area indicated at 0-11 largely due to dielectric loss.

Continuing to rise, thellquidl: reachesithe top-- of the columnand'overflowsatall points-around the circumference of, thetoptof thecontainer I .at voltages over 3600 or 4000.

voltages and frequencies depending on the materials undergoing treatmentand the character or treatment to be carried out.

A combination of resistance effect and dielectric effect takes placewhen electrode I4 is work ing against-electrode II or ground, thedistance between electrode I4 and the-side'of thecylinder or container 5regulating this dielectric effect. If the voltage impressed at thesurface of the filmis high enough, corona discharge also takes place.

As indicated above, the resistance and corona discharge effects may beaugmented by varying the character of the surface over which the thinliquid film flows. It may be metallized as indicated above or it may bean uneven surface as further described above or a combination ofnietallized uneven surface material may be employed.

-e processed liquid which flows over the dome E is collected in thetrough E and removed through the outlet i. If the hood member 8 isemployed any vapors which are condensed on that hood 8 may be removedfrom the collecting trough 9. Any residual vapors or gases may bewithdrawn through the exit it.

The whole unit or cell may be enclosed in a vacuum chamber in whichevent the apparatus may serve molecular still or low pressureevaporator. By applying a D. C. voltage through suitable circuits to thehood and film layer it is possible to use this force to move moleculesof vapor more rapidly across the space between them utilizing what maybe called a cathode sputtering effect.

If the liquid being fed in. falls in the class of a poor dielectric asdescribed above, it will heat more rapidly as a dielectric than by skineffect and corona discharge. Incidentally, it will be very hard toseparate the latter two effects, especially An example of such asolution would be a sugar solution which contains enough ionizable saltsto make it absorb power dielectrically at a speed in the class of 005%to 05% salt solutions. A pure sugar solution in pure distilled water isa very good dielectric and will not heat effectively by any of themethods disclosed. To heat sugar solution containing 123% salts theelectrode I3 would be spaced as close to the cylinder wall as possibleto prevent arcing, perhaps one inch or less at voltages of 10,000 orless, assuming the electrode is so designed as to have no sharp edges orcontours. If this electrode is so located as to boil the liquid just asit overflows in a thin film. much evaporates and concentration will takeplace, especially under a. vacuum, and because the boiling liquid andvapor above it are at a high potential due to proximity of the electrodeI3 and because they are better conductors when hot than when cold, andbecause vaporsabove slightly conductive solutions appear to act toinitiate invisible corona discharges, further heating effects areproduced at the surface and along the surface of the thin film. Voltagesimpressed on I3 shouldwbe on the order of- 10.000 or-more'ln downwardlyat an angle, spread outin a;-

order to raise the surface voltage high enough to get corona effects.

As the diameter of electrode I3 is expanded toward l4, and it couldpreferably be raised at the same time to be in proximity of undersurface 5 as it expands, the dielectric effect will become less at thesame electrode voltage and additional eflects of skin effectconductivity and corona discharge come into stronger play. Obtaininggreater effects from these phenomena is best done when the ionizablesalt concentration is such as to be too conductive for best and mostefficient heating dielectrically. The position of the ring electrodearound the cylinder insures some heating of the liquid body bydielectric effect, its proximity to the film area insures some coronaeffect, and the partially conductive film insures some leakage currentto flow along its surface to get to the ground via the body of liquid.Thus all three effects are useful here. At a point, I4, which may forexample, be 6 inches from 3, the dielectric effect may be rather smallat the same electrode voltage of 10,000 and the corona and skin effectheating will be predominant.

A further type of still or evaporator of a design i particularly suitedto heating by corona discharge effects is shown in Figure 3. It may benoted in this connection that liquids and solutions of higher electricalconductivity, heat faster from corona effect at any given voltage thando liquids of very low conductivity.

As shown in Figure 3, the reservoir 5 contains liquid l6 which flowsthrough the outlet ll therefrom into the passage or pipe l8, the lowerend of which is sealed as shown at I9 but is provided with a series ofopenings that permits the liquid to overflow downwardly along theoutside surface of the tube or th'imble 2|. During such flow downwardlyover the tube 2 I, the liquid is in the form of a thin film and eitheris vaporized or removed from the lower end of the tube 2| during whichtime it is subjected to heat treatment.

A high voltage, radio frequency electrode 22 connected through theconductor 23 to a suitable source of power, brings the power to thecenter of the heating surface indicated by the dotted lines 24. Suchsurface may be a glass surface because the liquid layer is usuallyconductive enough to act as its own electrode and to carry the potentialto all parts of the surface.

Unvaporized liquid collects in the receiver 25 and may be removedthrough the valve outlet 26. Gases or vapors which are formed may beremoved through the valved outlet 21.

The heating effect with this single electrode connection appears to bedue to the corona discharge eifect described above and partly to skineffect conductivity from the electrode contact point out to either endof the heating area. Steam forms rapidly when a slightly conductiveliquid flows over the surface of the tube 2| under suitable conditionsof voltage, rate of flow, and conductivity.

Many modifications of the described apparatus are possible, forparticular functions. A slight roughening of the outside surface of thetube 2| tends to help prevent breaks in the liquid film especially whena thin film is desired. Arcing and sparking may occur when the liquidfilm is not complete.

One method of assuring a substantially continuous film at almost anyrate of flow is to cover the surface of the tube 2| with a "wick offibrous material such as glass cloth in a manner similar to thatdescribed above in connection with the covering of the surface of thedome shaped member 5 over which the thin film of liquid flow in theapparatus of Figure 1. The very rapid evolution of steam with almost anydesired amount of liquid passing through without being evaporated as inthe concentration of solutions, may be attained at voltagessubstantially under 10,000 at 20 megacycle on dilute solutions ofelectrolytes such as sodium chloride.

This glass fiber wick" may be metallized as described above, if desiredto obtain other operating characteristics including catalytic effectswhich are characteristic of such metals as thin platinum films, silver,etc. The surface of the glass may also be metallized under the wickingif desired. 2

The receiver 25 has its upper cylindrical portion 28 carried upwardly toreceive the lower portion of the tube or pipe l8 and to form a seal at23. The neck portion 28 of the receiver 25 may be filled with materialeither in part or completely filling the space between the neck 23 andthe tube 2| to provide a surface over which the liquid may fiow in thinfilm form. Thus this space may be filled with a solid piece of sinteredglass having desirably fairly large pores, or helices, or other stillpacking or filling may be employed. This is illustrated in Figure '3,where the packing or filling 30 is shown between the neck 28 and thetube 2|. The utilization of such pieces of material gives much moresurface for vaporization or reaction in the same volume of space but, ofcourse, demands more power from the radio frequency generator.

In the device shown in Figure 3, an inlet 3| having a valve 32 may besupplied for passing in gases such as carbon dioxide or nitrogen formaintaining an inert atmosphere depending on the nature of the liquidundergoing treatment or for passing in a gas which is to react with theliquid that is flowing down in the thin film over the catalytic or plainsurface of the tube 2|. Unreacted gas or the gas introduced for purposesof creating the particular atmosphere within the apparatus may bewithdrawn as indicated above through the valved outlet 2'! and may berecycled to the inlet 3| if desired.

The features as to procedures carried out under reduced or increasedatmospheric pressures may be applied to the apparatus of Figure 3 justas in connection with that of Figure 1 as explained above. The apparatusis compact and readily made tight. Molecular distillation may be carriedout and all of the other features described above in connection with theapparatus of Figure 1 apply also to the apparatus of Figure 3.

There are many variables in setting up and operatin apparatus such as isdescribed in this specification but they will be readily recognized bythose skilled in the art from the disclosure herein. Thus the circuitsper se employed are those fully understood and used in the art, buthundred of modifications electrically, in them are possible. Thus somedielectric heating equipment is made with two high voltage outletconnections from a balanced or push-pull" circuit and others with onehigh voltage outlet to be worked against the ground. Two tubes arenecessary for the push-pull circuit and one or more in parallel can beused for the other. Both have strong supporters among radio men. Thechief advantages in this work is with one grounded electrode wherepossible because it increases operating safety factors in many cases. Inthe 7 case of Figure 1, the bottom electrode H, and

the feed-in line 3, and storage tank are safer by being at groundpotential.

This application is a continuation-in-part of application Serial No.626,860, filed November 5, 1945, entitled Heating Apparatus, and Method,now Patent #2485658.

Having thus set forth my invention, I claim:

1. Apparatus for heating a liquid that is a relatively poor conductorelectrically which poor conductor has the electrical conductingproperties of an aqueous sodium chloride solution of from '.04% to .10%and is a liquid that will not heat effectively as a dielectric becauseof its electrical conducting properties and will not heat effectively byinduction because of its electrical insulating properties, whichapparatus comprises a film forming member having a surface of extendedarea of a material that is conducive to surface heating effects in theliquid under conditions of use, a heating zone in which said memher isplaced, means for flowing said liquid as a film over the surface of saidmember of extended area, and means for subjecting said film in a highfrequency electric field to high frequency power at high voltage to heatthe liquid by skin effect phenomena, said last named means including asingle electrode only which electrode is adjacent said member ofextended surface area and through which power is supplied.

2. Apparatus as set forth in claim 1 in which the film forming memberhas an uneven surface affecting the invisible discharge characteristicsof said surface.

3. Apparatus as set forth in claim 1 in which the film forming memberhas a metallized surface affecting the invisible dischargecharacteristics of said surface.

4. Apparatus as set forth in claim 1 in which the film forming memberincludes a glass cloth over which the film of liquid flows duringheating.

5. Apparatus as set forth in claim 1 in Which the film forming memberincludes a body of packing material over which the film of liquid flowsduring heating.

6. Apparatus as set forth in claim 1 in which the member ofextended areais an elongated cylinder and the electrode encircles the cylinderbetween the ends thereof. 7

7. Apparatus for heating a material that is a relatively poor conductorelectrically which poor 12 conductor has the electrical conductingroperties of an aqueous sodium chloride solution of from 04% to .10% andis a material that will not heat effectively as a dielectric because ofits electrical conducting properties and will not heat effectively byinduction because of its electrical insulating properties, whichapparatus comprises means for moving said material in a relatively thinlayer through a heating zone, and means for subjecting said material ina thin layer in said heating zone in a high frequency electric field tohigh frequency power at high voltage to heat the material by skin effectphenomena, said last named means including a single electrode only whichelectrode is adjacent said first named means and through which power issupplied.

JOHN W. ROBERTSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 22,038 Milas Feb. 24, 19421,503,224 Blaine July 29, 1924 1,621,143 Vogel Mar. 15, 1927 1,934,704Golden Nov, 14, 1933 1,959,390 Smith May 22, 1934 2,023,637 KleinschmidtDec. 10, 1935 2,086,434 Rankin July 6, 1937 2,108,030 Darrah Feb. 8,1938 2,163,898 Van der Lande June 27, 1939 2,167,233 Dorcas July 25,1939 2,188,625 Dufour et al Jan. 30, 1940 2,303,341 Dufour et a1 Dec. 1,1942 2,336,542 Hatfield Dec. 14, 1943 2,384,982 Walton et al. Sept. 18,1945 2,397,897 Wenger Apr. 2, 1946 2,400,777 Okress May 21, 19462,427,094 Evans Sept. 9, 1947 OTHER REFERENCES Beck et al.: FilmCharacteristics, Industrial and Engineering Chemistry, November, 1936,pages 1251-1254.

Taylor: Heating Wood with Radio-Frequency Power, Transactions of the A.S. M. E, April, 1943, pages 201 and 202.

Industrial and Engineering Chemistry, May, 1944, pages 440-447,

1. APPARATUS FOR HEATING A LIQUID THAT IS A RELATIVELY POOR CONDUCTORELECTRICALLY WHICH POOR CONDUCTOR HAS THE ELECTRICAL CONDUCTINGPROPERTIES OF AN AQUEOUS SODIUM CHLORIDE SOLUTION OF FROM .04% TO .10%AND IS A LIQUID THAT WILL NOT HEAT EFFECTIVELY AS A DIELECTRIC BECAUSEOF ITS ELECTRICAL CONDUCTING PROPERTIES AND WILL NOT HEAT EFFECTIVELY BYINDUCTION BECAUSE OF ITS ELECTRICAL INSULATING PROPERTIES, WHICHAPPARATUS COMPRISES A FILM FORMING MEMBER HAVING A SURFACE OF EXTENDEDAREA OF A MATERIAL THAT IS CONDUCIVE TO SURFACE HEATING EFFECTS IN THELIQUID UNDER CONDITIONS OF USE, A HEATING ZONE IN WHICH SAID MEMBER ISPLACED, MEANS FOR FLOWING SAID LIQUID AS A FILM OVER THE SURFACE OF SAIDMEMBER OF EXTENDED AREA, AND MEANS FOR SUBJECTING SAID FILM IN A HIGHFREQUENCY ELECTRIC FIELD TO HIGH FREQUENCY POWER AT HIGH VOLTAGE TO HEATTHE LIQUID BY SKIN EFFECT PHENOMENA, SAID LAST NAMED MEANS INCLUDING ASINGLE ELECTRODE ONLY WHICH ELECTRODE IS ADJACENT SAID MEMBER OFEXTENDED SURFACE AREA AND THROUGH WHICH POWER IS SUPPLIED.